default to original ini setting if Cookie::$domain not set

We should not let PHP override the default setting if Cookie::$domain is not set. Otherwise it will send cookies with the host name of the server which generated the cookie.
commented comments to a multi line comment block

as suggested by @Zeelot
Merge pull request #447 from enov/bugfix/3604/Session_Native-Cookie-domain

Fixes #3604: default to original ini setting if Cookie::$domain not set